Why do my 3D prints curl and lift?

One of the most common problems for people starting out with their first 3D prints is getting the print to ‘stick’ to the build platform. Often you can’t even get the print to start correctly and you will be left with a ball of spaghetti. Other times you will get the print to start OK, but after a while the bottom of the print starts to curl and lift from the build platform. Eventually curling so badly, that the print gets knocked from its original position and you end up with a print where the bottom half is curled and twisted, while the top is a ball of spaghetti.

This is a companion discussion topic for the original entry at https://reprage.com/post/40834331256/why-do-my-3d-prints-curl-and-lift